In recent years, fuel cells have occupied an important position as next generation type clean energy sources. A solid polymer electrolyte used in a direct methanol type fuel cell (hereinafter referred to as a "DMFC") in which methanol is used as a fuel requires high proton conductivity and methanol barrier property.
As solid polymer electrolyte materials satisfying such requirements, hydrated membranes of perfluorosulfonic acid polymers such as Nafion (trade name) manufactured by E. I. du Pont de Nemours and Company) have generally been used. However, the hydrated membranes of perfluorosulfonic acid polymers have a limitation with regard to the methanol barrier property, because they are hydrated.
Besides, as a polymer having proton conductivity independent of water, polybenzimidazole (PBI) doped with a strong acid such as phosphoric acid (hereinafter referred to as "acid-doped PBI") is known. However, the acid-doped PBI membranes have the disadvantage that elimination of dopants such as inorganic acids is liable to occur in an atmosphere of water/methanol (liquid fuel). The present inventors have previously invented acid-doped PBI membranes in which dopant elimination is difficult to occur, and which are excellent in methanol barrier property by using diphenylphosphoric acid as a dopant in an amount of one molecule per N--H group in PBI (Japanese Unexamined Patent Publication No. 2000-38472).
For improving the low proton conductivity, a problem of the above-mentioned acid-doped PBI membranes, it is preferred that the N--H group density of base polymers is increased and that the density of acid components coordinated to the N--H groups is increased. Further, for conducting protons in the solid polymer electrolyte membranes, the base polymers preferably have a low glass transition temperature (Tg) and a flexible molecular structure. Furthermore, from the viewpoint of chemical stability required for the solid polymer electrolyte membranes used in fuel cells, the proton conducting polymers are preferably aromatic polymers.
The base polymers satisfying such requirements include polyanilines. The molecular structure thereof is simpler than that of PBI, and the N--H group density thereof is high.
The polyanilines include a polyaniline in which aromatic rings are bonded at the para-positions (hereinafter referred to as a "para type polyaniline"), and a polyaniline in which aromatic rings are bonded at the meta-positions (hereinafter referred to as a "meta type polyaniline").
It is known that the para type polyanilines are synthesized by various methods such as chemical oxidation and electrochemical oxidation. The physical properties of the para type polyanilines obtained vary depending on the synthesis method. For example, it is known that a polymer containing the para type polyaniline structure is obtained by polymerization of aniline in an aqueous solution of sulfuric acid in the presence of an oxidizing agent such as ammonium peroxodisulfate. In the production of the para type polyanilines, aniline, a starting material for dyes, is used as a raw material, so that they are produced at low cost. The para type polyaniline has a .pi. conjugate structure, so that the para type polyaniline itself has electrical conductivity, and of the conductive polymers, it is relatively high in stability. Accordingly, although an acid-doped para type polyaniline obtained by doping the para type polyaniline with an acid component shows proton conductivity, it can not be used as a material for the solid polymer electrolyte membrane used in the fuel cell.
On the other hand, the meta type polyaniline can not have a .pi. conjugate structure, so that it can not exhibit electrical conductivity as it is.
However, the meta type polyaniline has proton selective permeability (proton conductivity). Accordingly, there is an example in which it is evaluated as a PH sensor usable in metal ion-containing solutions [Onuki, Matsuda and Koyama, Nippon Kagaku Kaishi, 11, 1801 (1984)]. Further, the meta type polyaniline has a flexible molecular structure, compared with the above-mentioned para type polyaniline. Like this, the meta type polyaniline having no electrical conductivity and having the flexible molecular structure is anticipated to exhibit the proton conductivity by doping with an acid component (hereinafter referred to as "acid doping"), when the acid doping is possible, and the application as a novel solid polymer electrolyte material for fuel cells is expected.
Furthermore, as an electrode used in a solid polymer electrolyte type fuel cell, a so-called MEA (membrane electrode assembly) is known. In the MEA, electrodes are formed of fine catalyst particles prepared by allowing carbon to support a noble metal, a solid polymer electrolyte component formed on surfaces of the fine catalyst particles, and a fluorine resin for adhering the fine catalyst particles to one another. The electrodes are each arranged on two main planes of a solid polymer electrolyte membrane, thereby constituting a fuel cell (Japanese Unexamined Patent Publication No. 5-36418).
The polyaniline and the acid-doped polyaniline (hereinafter referred to as an "acid-doped polyaniline"), that is to say, the proton conducting polymers, are used as the solid polymer electrolyte components formed on the surfaces of the fine catalyst particles, when they are high in proton conductivity, and they are expected to be used as novel electrode catalysts for fuel cells.
However, for synthesis methods of the meta type polyanilines, only a few examples of electrolytic polymerization of aniline under special conditions are reported [T. Ohsaka et al., J. Electroanal. Chem., 161, 399 (1984), A. Volkov et al., J. Electroanal. Chem., 115, 279 (1980), and Onuki, Matsuda and Koyama, Nippon Kagaku Kaishi, 11, 1801 (1984)]. Thus, the development of novel synthesis methods of the meta type polyanilines used for various applications has been expected.